I use another camera, Fujifilm S3000.

This is a good camera, and is more suited to the LEGO purpose. You see, it has a larger lens that allows more light to enter. THis makes the auto focus more effective, I believe. So, get a camera like mine. I also have a camera similar to yours, which I use for trips as it is more portable.
And get a real tripod.

Mine is similar to that one, couldn't be bothered to seek out a pic of my real one...
Timer also works, but not as good. A combo is the best.
The LEGO tripod you use can bend under the weight of your camera. A real tripod is much better for that. Also, any more equipment you can find should be good. But I guess all of my stuff sounds sorta too expensive... But it's worth it:

You also need good light. Daylight is awesome, but there are certain types of lamps you could use... But you need white light, and a lot of it. That way, you can use autofocus more effectivly. In low light conditions, increase the time your film is exposed to the light, it increases the brightness. This also works when taking pictures of the night sky.
Also, you need a background. It's the one thing one shouldn't notice in a MOC. I use white, green, blue or red A1 (I think) paper. It is a cheap way to get a nice background. It has no edges so you can edit it into a completly white background. Here's the result:

So, you now have good pics! Not to edit them!
Use GIMP. Free and awesome, you have it allready. To edit it, sharpen it a bit and play with the contrast and brightness.

I hope that's the pic type you want, tho I didn't contrast it enough.
On the other hand, close ups can be made either in macro mode or with the zoom tool.